Filtration processes are used to separate compounds of one phase from a fluid stream of another phase by passing the fluid stream through filtration media, which traps the entrained or suspended matter. The fluid stream may be either a liquid stream containing a solid particulate or a gas stream containing a liquid or solid aerosol.
For example, filters are used in collecting dust emitted from incinerators, coal fired boilers, metal melting furnaces and the like. Such filters are referred to generally as “bag filters.” Because exhaust gas temperatures can be high, bag filters used to collect hot dust emitted from these and similar devices are required to be heat resistant. Bag filters can also be used in chemically corrosive environments. Thus, dust collection environments can also require a filter bag made of materials that exhibit chemical resistance. Examples of common filtration media include fabrics formed of aramid fibers, polyimide fibers, fluorine fibers and glass fibers.
Polyphenylene sulfide (“PPS”) polymers exhibit thermal and chemical resistance. As such, PPS polymers can be useful in various applications. For example, PPS can be useful in the manufacture of molded components for automobiles, electrical and electronic devices, industrial/mechanical products, consumer products, and the like.
PPS has also been proposed for use as fibers for filtration media, flame resistant articles, and high performance composites. Despite the advantages of the polymer, however, there are difficulties associated with the production of fibers from PPS. PPS fibers typically have poor mechanical properties. Accordingly PPS fibers do not have sufficient tensile strength for many applications. In addition, PPS fibers are brittle and thus are not readily manufactured into fabrics for use in downstream applications.
Prior attempts to improve the mechanical properties of PPS fibers have met with limited success. PPS has been blended with another polymer and the blend meltspun to produce monofilaments. The blend monofilaments, however, do not necessarily overcome the problems associated with the poor tensile strength and brittleness of PPS. Further, the blend monofilaments can exhibit a small improvement of one property to the detriment of another property. A monofilament, with its relatively large diameter, would also be inherently less effective in a filtration medium than a smaller diameter fiber.
Still further, the problems of producing PPS blend fibers are compounded by the limited compatibility of PPS with other polymers. A compatibilizing agent typically is required to make the fibers in the first place. Yet this can compromise the desired fiber properties and add additional processing steps and costs to fiber production.
Another approach is to mix mineral fillers or reinforcing fibers with the PPS polymer to provide sufficient strength to products produced from the PPS material. However, such blends cannot be used for fiber extrusion because of the presence of the mineral fillers and/or reinforcing fibers.
U.S. Pat. No. 5,424,125 to Ballard et al. is directed to monofilaments made of polymer blends, namely, a blend of PPS and at least one other polymer selected from polyethylene terephthalate, high temperature polyester resins, and polyphenylene oxide (PPO). The polymers of the blend are present throughout the cross section of the fiber, so that the exterior surface of the fiber includes polymers in addition to PPS. This in turn can limit the usefulness of the resultant fibers in severe service high temperature and/or corrosive environments. Further, while the Ballard et al. patent indicates that a compatibilizer is not required, the patent describes the use of compatibilizers in the production of the fibers. In addition, the Ballard et al. patent requires a large amount of polymer other than the PPS polymer, and in particular at least 50 present by weight, and higher.
Published Japanese Application 03104924 is directed to conjugate fibers stated to have good dyeability. The fibers include a polyphenylene sulfide polymer layer and a protecting layer. The protecting layer, formed of a polymer other than PPS, is required to be present on an outer surface of the fiber to impart dyeability thereto. Otherwise the fiber would not be dyeable. The resultant fiber is subjected to an oxidizing treatment using, for example, hydrogen peroxide, to oxidize the PPS. The publication indicates that the fibers must be oxidized, otherwise the fibers will not perform as required.
Other published Japanese applications discuss the production of PPS fibers. Generally the fibers include at least one polymer in addition to PPS on the outer surface thereof so as to impart desired properties to the end product. Yet, the presence of polymers other than PPS on the fiber surface compromises the properties imparted thereto by PPS. Also, generally the fibers require the presence of additional materials incorporated into the fiber, such as an electrically conductive material, an adhesion promoting agent, such as a tie layer between sheath and core components, and the like. Yet this can increase the complexity and cost of fiber production.
JP 3040813 describes fibers with a polyamide core component in combination with a PPS sheath component. As noted above, however, PPS exhibits limited compatibility with other polymers. This lack of compatibility is further exacerbated with polyamides, which generally do not adhere well to other types of polymers.
There have been attempts to improve the adhesion and/or compatibility of polyamide with PPS using various adhesion promoting techniques. For example, JP 4343712 describes a fiber including a component formed of a blend of polyamide with PPS. JP 4327213 describes a fiber with a modified PPS sheath in which the PPS includes maleic anhydride. See also JP 2099614, describing a fiber including a polyester/PPS blend core component and a PPS sheath component. Yet such techniques can increase the cost and complexity of fiber production and further can compromise fiber properties, particularly for fibers modified to include a polymer other than PPS exposed on the surface thereof.
JP 6123013 and JP 5230715 propose composite fibers including an anisotropic, e.g., a liquid crystalline polymer, component and a PPS component. Liquid crystalline polymers, however, can be expensive and difficult to melt spin, thereby also increasing the cost and complexity of such fibers.
U.S. Pat. No. 5,702,658 to Pellegrin et al is directed to a rotary process for the production of bicomponent fibers. The rotary process, similar to that used in the production of glass fibers, is stated to be useful in the production of fibers using polymers with varying physical properties, such as different viscosities. The rotary process uses centrifugal force to attenuate the fibers, in contrast to the mechanical attenuation of conventional fiber extrusion processes. For polymers with different viscosities, the centrifugal force wraps the low viscosity polymer about the higher viscosity polymer so that the interface between the two is curved.